1. Field of the Invention
The present invention relates to an optical member, to a surface light source unit, and to a liquid crystal display. In particular, the present invention relates to an optical member including optical sheets layered on each other, to a surface light source unit including this optical member, and to a liquid crystal display including this surface light source unit.
2. Background Art
In recent years, liquid crystal displays using liquid crystal display panels (hereinafter also referred to as LCD panels) have been used in diverse fields as means of displaying images for use in personal computers, television sets, various information display terminals, and so forth. In such a liquid crystal display, the LCD panel is of transmission type. A surface light source unit converted light from a source to surface light (planate light) so as to illuminate the liquid crystal display panel from the backward side.
Surface light source units are roughly classified into two types, light-guide-plate type (including edge light type and side light type) and direct type. In a surface light source unit of light-guide-plate type, light sources such as cold cathode fluorescent lamps are arranged along the side edge of a flat light guide plate made from acrylic resin or the like excellent in light transmission properties. Light from the light sources travels in the light guide plate and lastly exits from the light guide plate. On the other hand, in a surface light source unit of direct type, no light guide plate is used, and light sources are placed in such positions that the light sources can face a liquid crystal display panel, with a plurality of optical sheets between the light sources and the liquid crystal display panel. Surface light source units of light-guide-plate type, with which decrease in power consumption and in the thickness of displays can be achieved, are mainly used for liquid crystal displays in small sizes of 20 inches or less. Surface light source units of direct type, on the other hand, are mainly used for large-sized liquid crystal displays.
FIG. 5 illustrates a conventional surface light source unit of direct type. A surface light source unit 40 shown in FIG. 5 comprises an optical member 20 capable of transmitting light, light sources 31 arranged on the light-entrance side of the optical member 20 so as to face the optical member 20, and a reflector 32 situated so as to face the optical member 20, with the light sources 31 between the reflector 32 and the optical member 20. The optical member 20 comprises a diffusing layer 21 made from styrene, acrylic resin, or the like, an adhesive layer 22, a light shielding layer 23 having openings, and a lens sheet 24, which are situated in the order named with the diffusing layer 21 in the outermost position on the light source 31 side. The above layers making up the optical member 20 are usually formed integrally. Such a surface light source unit 40 and an optical member 20 are also disclosed in Japanese Laid-Open Patent Publication No. 2007-47257.
Furthermore, an optical member 25 as shown in FIG. 6 has recently come to be used as well. The optical member 25 shown in FIG. 6 comprises a diffusing sheet 26, a brightness accentuating film BEF (trademark, Brightness Enhancement Film) 27 available from 3M Corporation, USA, and a polarized-light-splitting film DBEF (trademark, Double Brightness Enhancement Film) 28 available from 3M Corporation, which are laminated in the order named with the diffusing sheet 26 in the outermost position on the light source 31 side. Although these layers (the sheet and the films) are not stuck to each other, they are held in the state of being layered on each other.
In the optical sheet BEF 27, unit prisms that are triangular in sectional form and that are much larger in size as compared to wavelengths of light are regularly arranged on a base in sheet form. Such an optical sheet 27 can makes it possible to use light from light sources efficiently, since this optical sheet 27 can collimate off-axis light so as to redirect the light to the on-axis direction towards viewers or so as to recycle the light.
However, when incorporated in a liquid crystal display, such an optical member tends to be warped to be concave relative to the light source 31 side (light-entrance side) and convex relative to the liquid crystal display panel 19 side (light-exit side) while the light source are on. The optical member 25 thus presses the liquid crystal display panel 19 even to such an extent that a displayed image is adversely affected.